(1) Field of the Invention
The present invention relates to a fluorescent-lamp driving apparatus that performs driving using a high-frequency inverter method, and a compact self-ballasted fluorescent lamp equipped with such fluorescent-lamp driving apparatus.
(2) Related Art
Recently, the energy-saving trend has started to prevail in the field of illumination. Accordingly, compact self-ballasted fluorescent lamps are replacing incandescent lamps having been conventionally used.
A representative compact self-ballasted fluorescent lamp is composed of: an arc tube fixed to a holder; a driving apparatus for driving the arc tube; and a case provided so as to keep the driving apparatus from such as human hands at the time of driving. At one end of the case, a base is fixed for fixing the compact self-ballasted fluorescent lamp to a socket, and for taking in power from the commercial electric source.
At both ends of the discharge path of the arc tube, electrodes made of filament coil are provided, and each lead therefrom is connected to the driving apparatus.
Conventional driving apparatuses are magnetic circuit type such as a glow-start type and a rapid-start type. However recently, those adopting inverter method have started to be used, because of advantages in realizing production of smaller devices, and in reduction of energy loss. A representative driving apparatus adopting inverter method is composed of: a rectifier circuit that includes a diode bridge device and an electrolytic capacitor device; a resonance circuit made of a choke coil, a resonance capacitor device, and the like; and an inverter circuit whose main components are two FET devices.
In such an inverter-type driving apparatus, electrodes in the arc tube will be preheated for a given time period, at the driving start (i.e. when the power is turned on). As the temperature of the electrodes rises, the frequency of the inverter circuit will gradually go down, and the resonance circuit composed of the choke coil and the resonance capacitor device will have increased voltage, at the driving apparatus. The discharge will start in the arc tube when the voltage of the resonance capacitor device becomes higher the starting voltage of the arc tube (e.g. Japanese Patent Publication 2002-75010).
Incidentally, when the arc tube reaches the end of the life, a blowout occurs at a portion of the filament coil, thereby leading to so called “non-emission” state. In the non-emission state of a compact self-ballasted fluorescent lamp, the lamp voltage (Vla) becomes high, and the tube electric current becomes small. Therefore large electric current will run via the preheating capacitor, so as to have the choke coil and the FET devices to generate heat. If this state continues, the temperature of the electrodes and the surrounding area will gradually increase. There sometimes happens that the temperature of the electrodes and the surrounding area reaches to an extent that melts the case made of resin.
Therefore, it is important, in compact self-ballasted fluorescent lamps, to stop the glow discharge to be generated at the electrodes in the non-emission state, swiftly and assuredly.
In view of this, the compact self-ballasted fluorescent lamp having the conventional inverter-type lighting circuit (such as the Japanese Patent Publication 2002-75010) adopts the following method. That is, when such a non-emission state results, the increase in electric current running in the circuit breaks the FET device, so as to stop the switching device function of the FET devices, and further to stop the glow discharge from the electrodes. To be more specific, if the compact self-ballasted fluorescent lamp results in a non-emission state, a blowout of the electrodes will raise the lamp's lighting maintain voltage. In response to this, the voltage applied to the resonance capacitor device will increase. This leads to increase in electric current to be supplied to the electrodes (i.e. increase in electric current running in the FET devices of the driving apparatus), so as to have the FET devices to generate heat. The FET devices will break when this electric current exceeds the upper limit for the FET device. This is how the FET devices cease to function as a switching device.
Incidentally, recent trend with the compact self-ballasted fluorescent lamps is to use a spiral arc tube, instead of conventional U-shape arc tubes, because spiral arc tubes have excellent luminous efficiency. However, if such a compact self-ballasted fluorescent lamp equipped with a spiral arc tube has a conventional driving apparatus, glow discharge at the electrodes cannot be stopped swiftly and assuredly, in a non-emission state. This is because, with a compact self-ballasted fluorescent lamp with a spiral arc tube, only small electric current runs to the FET device, compared to a counterpart with a U-shape arc tube and the like. This means that, in the compact self-ballasted fluorescent lamp with a spiral arc tube, the FET device will not be broken over a long period of time after the non-emission state begins.
In view of the above, in the compact self-ballasted fluorescent lamp, particularly in the compact self-ballasted fluorescent lamp equipped with a spiral arc tube, it is desired to stop glow discharge to be generated at the electrodes of the arc tube in the non-emission state, more swiftly and assuredly than conventionally.